It is known that in the most demanding applications for fiber optic interferometric sensors, such as geophones, hydrophones and distributed Coherent Rayleigh (CR) effect based sensors require ultra-low frequency noise laser sources. Ultra-low frequency noise laser sources are also required in other applications, such as, precision navigation and atomic clocks. Industrial applications such as continuous wave (CW) coherent Doppler LIDAR and remote Laser Doppler Vibrometry (LDV) require ultra-low excess noise contribution, i.e. very narrow Lorentzian linewidth laser sources with linewidth below 1 kHz. The most widely used approach for frequency noise reduction is to utilize electronic feedback frequency control, which is primarily used with distributed feedback (DFB), DFB-fiber and semiconductor external cavity lasers.
There are a few major requirements to achieve significant frequency noise reduction using electronic feedback control based on the optical frequency discriminators.
First, free running lasers must have low frequency noise to begin with, i.e. thre lasers must be qualified as a “sensing” laser or “narrow-linewidth” laser. Examples of such lasers are a fiber lasers for sensing applications such as developed by Koheras Inc, Orbits Lightwave Inc, NP Photonics, Inc and PLANEX type semiconductor external cavity lasers developed in Redfern Integrated Optics, Inc. of Santa Clara, Calif.
Secondly, optical discriminators that convert laser frequency noise δv to voltage, noise δV, must have large sensitivity (δV/δv) for such conversion. Examples of such optical discriminators are Fabry-Perot (FP), Mach-Zehnder (MZ), and Michelson interferometers or any other type of reference stabilized cavity. Choice of discriminator depends on the ability of linear conversion of frequency noise to the voltage noise (known as “quadrature conditions”). Requirements include continuous operation of frequency stabilized laser without reset, packaging stability and dimensions, ease of fabrication and the inherent sensitivity to environmentally induced frequency drift.
Another important consideration is how a bandwidth (BW) of frequency feedback loop is limited by the laser properties i.e. how an electronic error signal is applied to a laser source. Low frequency noise semiconductor laser sources, such as DFB-fiber laser with piezoelectric PZT tuning section, DFB lasers, ECLs with semiconductor gain elements, etc. have such inputs applied as a bias current. Various references, such as, US Patent publication 2010/0054288, US Patent Publication 2006/0159135 etc. described such frequency stabilized laser sources with different choice of optical discriminators, and different architecture of electronic feedback control. Regardless of all differences in the laser sources and feedback architecture, there is one common way of applying error frequency signal in all the references: error frequency signal is applied to the bias current input of semiconductor gain element.
The inventor of the present application has recognized that such approach represents fundamental limitations on frequency noise reduction as described below.
When a current error signal is applied at particular frequency to the laser current bias, there is an associated phase change. However, when phase change is below so called phase reversal (i.e. when the phase changes by 180 degrees), sign of the feedback remain unchanged, i.e. feedback remains negative. In DFB sources such phase reversal frequency is between 300 kHz and few MHz. In the external cavity lasers such phase reversal frequency is usually below 100 kHz. Particularly for PLANEX type lasers, such frequency is below few hundred kHz. Operating bandwidth (BW) of a system is often dictated by the phase reversal. Bandwidth of the feedback control circuitry plays the most important role in frequency noise reduction. The residual phase-noise variance in close loop feedback operation is inversely proportional feedback bandwidth, σφ2˜Δv/BW.
Accordingly, what is needed is an implementation of a feedback control circuitry that ensures stable operation of an external cavity semiconductor laser for applications that demand low frequency-noise.